Inverters are often used in power conversion systems to produce AC power. For example, in a variable-speed, constant-frequency (VSCF) power generating system, a variable-frequency AC output of a brushless, synchronous generator is converted into DC power by an AC/DC converter and the resulting DC power is converted by an inverter into constant-frequency AC output power. Typically, such inverters are of the three-phase type wherein three-phase output voltages are produced by the inverter. In a VSCF system of this type, a voltage regulator detects the average of the phase output magnitudes and delivers a controlled current to an exciter of the brushless generator to in turn control the output voltage of the generator, and hence the average of the three-phase voltages produced by the inverter.
Occasionally, an unbalanced condition can arise in loads connected to the inverter output. This, in turn, can cause the output voltage magnitudes to become unbalanced or substantially unequal. Such a condition is undesirable where output voltage magnitudes must be held to within a certain range of a specified level. During an unbalanced load condition where one of the output voltage magnitudes decreases a substantial amount relative to the remaining two output voltages, the three-phase output voltages will be increased by the voltage regulator so as to maintain the average of the magnitudes at the specified level. This does not, however, cause the differences in level between the individual phase voltages to be reduced so that they are within a specified range.
Corey et al., U.S. Pat. No. 3,354,376 discloses a multiphase inverter which produces three-phase output voltages. The inverter includes a pair of single-phase inverters which are coupled to a Scott-T transformer having three outputs at which the phase voltages are produced. A regulator includes circuits which sense the voltage amplitudes between the three phases. Two of the sensed voltage amplitudes are used to control the amplitudes of the outputs of the single-phase inverters and the remaining sensed voltage amplitude is used to control the phase displacement between the outputs of the two inverters. By controlling the amplitudes and phase displacement in this fashion, the three phases may be regulated and balanced.
As should be evident from the foregoing discussion, the Corey system is capable of only regulating a three-phase output produced by single-phase inverters coupled to a Scott-T transformer. Since such types of polyphase inverters are not in wide usage currently, this system is of limited usefulness, at best.